Symposium Looks at History of, Threats to, and Hopes for Hawai`i's Remnant Dry Forests

posted in: April 2013 | 0

Peter Vitousek didn’t mince words: “Everything we’ve lost, and everything we will lose, is because of human activity. We’re responsible for that change.”

Vitousek, a professor of biological sciences at Stanford University, made the comments as the opening speaker at the seventh annual Nahelehele Dryland Forest Symposium, held March 1 at the King Kamehameha Hotel in Kona. Addressing the crowd of scientists, resource managers, volunteers, and others concerned with protecting these rare ecosystems, Vitousek linked the decline of dry forests in Hawai`i to their loss globally and sought to inspire those in attendance with “something of a revival meeting to remind us of the importance of what we’re doing.”

“These forests are widespread,” he noted. The conditions that occur in Hawai`i that led to the growth of dry forests occur elsewhere in the world, he said, adding that their very definition “implies several features that make them vulnerable:

“Their soils are typically more fertile than soils of rainforests. That makes sense, because soil fertility is governed in part by how much water passes through soil, leaching nutrients out….

“Also, the long dry season means fire can be used in forest clearing. It’s a lot easier to clear a forest if you can use fire, even though some forests are quite resistant to fire if they’re intact.

“Long dry seasons help cultivators control weeds and pests, so they’re better suited to intensive agriculture….

“Disturbance and seasonal variation support more biological invasions. Speaking globally, … a really widespread invader is tropical grasses.”

As a result, Vitousek said, “what you see if you look at deforestation globally is arcs of deforestation that are taking place around the great rainforests – not the centers of rain forests but their drier margins, which are more suitable than fain forests for clearing, easier to clear, easier to keep clear, and more suited to various kinds of human land use… What’s left are rocky sites too hard to farm once you’ve cleared them … and very small protected areas.”

Vitousek pioneered the study of rain-fed agricultural systems in North Hawai`i, particularly on the leeward slopes of Kohala mountain where the miles-long walls, the remains of thousands of agricultural fields, still show up clearly in aerial photographs. “These areas were farmed very intensively by Hawaiians,” he noted, especially areas receiving from 30 to 70 inches of rain annually.

“It was mesic to dry forest when the Hawaiians began cultivating it,” he said. The soil in these areas is “extraordinarily rich compared to either wetter or drier areas, and it got so rich because the forests that grew there before clearing were extremely deep rooted… They persist through the dry season by having deep roots, and in the process of pumping up deep water they pump up nutrients. What makes this area so rich is tens of thousands of years of them pulling nutrients from the soil and depositing them on the surface.”

Through carbon dating of charcoal deposits found in a trench excavated by Vitousek’s alma mater, Hawai`i Preparatory Academy in Waimea, it was possible to obtain an idea of just how recently the dry forests in the area were cleared.

“The maximum age for this charcoal was 450 years,” Vitousek said. “This was a dry forest cleared for agriculture in this area of high soil fertility, created by the dry forest itself. It wasn’t something killed by rats. It is something that only 450 years ago or less, when agriculture was being intensified in the place, was cleared by Hawaiians.”

Setting the dry forests of Hawai`i island apart from those on other islands was the young age of the land itself. “These dry forests that created the sweet spot of soil fertility that Hawaiians found when they cleared and cultivated them … were rich enough for intensive agriculture and low enough in elevation so Hawaiian crops could grow,” he said. “We also have beautiful dry forests on Kaua`i, in the Wai`anae range, and in areas that get 50 inches of rain or less a year. They have deep soils, but the fertility is much less than elsewhere. They’re tremendously infertile. That’s because they have been sitting there for thousands of years, they’ve had nutrients pumped out by the dry forests for millions of years. So those areas are not really suitable for intensive agriculture, as are the newer soils of Kohala.”

“The dry forests we have now, the dry forests we’re working with here, are beautiful and spectacular,” Vitousek said. “But they’re not the universe of dry forests that were here before. They’re remnants. There are parts of that universe we just don’t see anymore. And that’s all the more reason to keep the ones we have, to do the work we’re doing.”

“The efforts going on here at Ka`upulehu or Mahaulepu or Kanaio or Auwahi, where all of you are working to keep these dry forests alive or to restore them, are on the leading edge of a global challenge…. It’s still the anthropocene era, but the anthropocene can be an era of stewardship as well as an era of loss…. What we do, how we work to sustain these forests is something the world can use as well.”

A Tortoise Monastery on Kaua`i 

Vitousek spoke of the anthropocene era, one in which the human species has emerged as a significant force capable of reshaping the face of the Earth.

But on a small patch of land along Kaua`i’s southern coast, tortoises have become the agents of change.

David Burney, a paleoenvironmentalist with the National Tropical Botanical Garden, has rescued abandoned giant tortoises from across Hawai`i and put them to work clearing weeds from an area where he and others are attempting to restore a dry coastal ecosystem.

Much of Burney’s work has been below ground, in Makauwahi cave, the largest limestone cave in Hawai`i and possibly the richest fossil site in the entire Hawaiian chain. “We realized early on, working with Helen James and Storrs Olsen … that the feeding guilds that are missing are the predators, the terrestrial herbivores, the terrestrial omnivores. They’re almost entirely absent from native habitats today,” he said. “That started us thinking, particularly about herbivores.”

One of the largest of these birds, he continued, was the turtle-jawed moa nalo. “It truly had a tortoise-like beak … and would have been the biggest thing around on land. They probably had no natural enemies as adults because they were so much bigger…. So we started thinking about that.”

Then, on a visit to the island of Rodrigues, in the Indian Ocean, Burney observed a restoration effort that avoided what he called the “weed treadmill” that so often discourages restoration efforts in Hawai`i – where no sooner are weeds pulled than another one takes its place. As soon as an area is planted in Rodrigues, however, “they fence the area, put in giant tortoises, and walk away. The tortoises do the weeding and fertilizing, and germinate seeds by eating fruits.”

Burney noticed also the presence of several native Hawaiian plants, which, he said, “had become mildly invasive.”

That got Burney and his colleagues thinking about the “toothless islands” of the West Indies, Melanesia, and Hawai`i. “Our plants are not defended against animals with teeth and advanced stomachs,” he said. “But they do have defenses against creatures with beaks, toothless creatures.”

Tortoises, it turns out, “have no interest in native plants,” Burney said. “They have something about them that they don’t like, in terms of defense compounds, growth habits, et cetera.”

“One year and eleven giant tortoises later,” he continued, “we cast our lot with the Humane Society and the Turtle Conservancy. We discovered there are hundreds of giant tortoises right here in Hawai`i already. They got here through the process of looking cute in pet stores.”

The animals are unlikely to become invasive pests, Burney said. “They can’t breed here. The temperature of the soil is too low for them, and if any of the eggs managed to hatch, they’d all be males anyway. We’re running a tortoise monastery.”

The worst that tortoises can do to native plants is to crush them. “If you plant natives, let them get up to some size” before unleashing the tortoises on them, he said.

Only one weed seems resistant to tortoises – the thorny Mimosa pudica, also known as the sleeping plant for its habit of curing up its leaves in response to touch. The tortoises aren’t put off by the thorns, Burney said, and are happy to munch on it when it is already dead. Rather, they seem to be spooked by the sudden movement of the living plant.

When Burney’s talk was finished, one member of the audience raised the question that seemed to be on everyone’s mind: “How many tortoises per acre are needed to keep the weeds down?” Burney was asked. It depends on the size of the tortoises, he replied, noting that they continued to grow throughout their lives. But, if you have medium-sized tortoises, about six of them per acre should keep the weeds at bay, he answered.

Threatening Thrips

A new-to-Hawai`i species of thrips is preying on the native naio, or false sandalwood (Myoporum sandwicense), with a mortality rate that is up to 90 percent in some areas.

That was the grim report from Cynthia King, an entomologist with the Department of Land and Natural Resources’ Division of Forestry and Wildlife.

The thrips was first observed in the winter of 2008 by landscapers on the Big Island, she said, and was officially reported by the Hawai`i Department of Agriculture in 2009. In May of that same year, the DOA stopped the inter-island movement of naio, but by then it was already widely spread across the Big Island.

The species was first detected in 2005 in California, where it continues to devastate Myoporum trees. “At the time it was discovered in California and Hawai`i, we didn’t know where they were from,” King said. “We suspected the Australia-New Zealand region, since that’s where Myoporum diversity is highest.”

Eventually, researchers tracked the thrips to Tasmania. “So, from Tasmania it went to California, and from California to here,” King said, almost certainly in a shipment of landscape material.

The thrips affects the growing terminals of the tree, King noted, and infestation rates are increasing across all sites where the thrips is found. And as the infestation rates increase, “so, too, are dieback rates” on the rise, she added.

King listed some of the potential impacts if the thrips is not controlled. One of the biggest impacts, already being seen in dry forest restoration efforts on the Big Island, is the exclusion of naio from replanting schemes. “Naio has been a great go-to plant as far as outplanting and restoration efforts are concerned,” King said, “but I know on the Big Island, folks are already adapting their strategies.”

Other possible impacts cited by King include:

  • Alteration of already threatened habits (coastal strands and dry forests especially);
  • Loss of fauna that is dependent on naio (pollinators, herbivores, seed predators, and borers); and
  • Loss of forest and habitat structure (for example, naio is co-dominant with `ohi`a in critical habitat for the endangered palila).

For landscape-scale control, finding a biocontrol agent is the only feasible option, King noted. But so far there has been no international effort to look for natural enemies.

Hawai`i does have one endemic species of a tiny parasitoid wasp that preys on native species of thrips, King said. It is being studied to see if it might prey on the Myoporum thrips as well.

In the meantime, “our focus is on early detection of the thrips in the outer islands, pursuing funds for additional biocontrol research, and consider seedbanking efforts, similar to those for wiliwili” when it was under attack by the wiliwili gall wasp.

And a Success Story

Speaking of the wiliwili gall wasp, Leyla Kaufman, with the Department of Plant and Environmental Protection Sciences at the University of Hawai`i at Manoa, gave an update on what has occurred since release of a biocontrol agent for the pest.

Back in 2005, the gall wasp spread through the islands like wildfire, leaving devastated trees in its wake. Thanks to a Herculean search by the Department of Agriculture’s exploratory entomologist, Mohsen Ramadan, a parasitoid wasp was found in eastern Africa and eventually approved for release in Hawai`i in 2008. Since then, it has been controlling wiliwili infestations.

But the infestations have not been eliminated altogether, Kaufman reported. Teams of researchers from the DOH, the DLNR, and the University of Hawai`i surveyed sites across the islands both before and after the release to obtain a good idea of how effective the parasitoid wasp, Eurytoma erythrinae, has been in suppressing the gall wasp. Before the release, high infestation rates were found on young shoots of wiliwili trees. By 2012, rates of infestation were generally low. Still, she added, “more than 40 percent of inflorescences are heavily infested. So, while there’s been a marketed improvement in infestation of new shoots, there’s still high infestation in the flowers.”

Among the areas that remain hardest hit by the gall wasp is the Waikoloa dry forest on the Big Island.

Trees may still produce seeds even when infested at a high level, Kaufman said, although researchers did find a “significant correlation between germination rates and infestation rates; clean seed pods have a higher germination rate.”

Not every tree survives an infestation. “Last year we conducted a census of wiliwili populations,” Kaufman said. “We found that of 518 trees, 30 to 40 percent died due to gall wasp infestation. Smaller trees were more vulnerable than larger ones.”

All in all, she concluded, “Eurytoma established itself fast and has done a good job keeping infestation rates in leaves controlled, but the infestation in inflorescences is still high.” One of the most ominous observations, she noted: “No recruitment was observed at most sites. At almost all sites, we haven’t seen any new keiki.”

But she stopped short of laying blame at the foot of the gall wasp.

“This is mainly due to competition with invasive weeds,” she said.

In an effort to target infestation in flowers, the DOA is working on another biocontrol agent, she said.

Patricia Tummons

Volume 23, Number 10 — April 2013

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